1. Field of the Invention
The invention relates to a differential gear. More particularly, the invention relates to an improvement of a differential gear in which a first differential case that supports a differential mechanism and transmits torque, a second differential case that does not transmit torque to and from the differential mechanism, and a ring gear are all formed separately and integrally joined together by welding. The invention also relates to a vehicle provided with this differential gear.
2. Description of the Related Art
One differential gear in which a differential case is divided into two cases, i.e., a first differential case and a second differential case, in the axial direction, with a differential mechanism that includes a pinion gear and a pair of side gears or the like housed inside the differential case and a ring gear provided on the outer portion of the differential case, and in which the first differential case, the second differential case, and the ring gear are all integrally joined together by welding, is used as a differential gear unit that allows differential operation of left and right wheels of a vehicle, for example. This is because with a four-pinion type differential gear, a forged differential gear, or a limited-slip differential gear (LSD), for example, it is necessary to divide the differential case into two due to structural limitations as well as for assembly reasons and the like. Japanese Patent Application Publication No. 7-54960 (JP-A-7-54960) describes one example of such a differential gear. In this differential gear, the joining portions of the first differential case and the second differential case are overlapped with each another and joined together by welding. A ring gear is then integrally joined by welding to the outer peripheral side of the portion where the joining portions are joined together, and a pinion shaft is attached to the inner peripheral side of the portion where the joining portions are joined together. Also, Japanese Patent Application Publication No. 2006-509172 (JP-A-2006-509172) describes technology in which a first differential case to which a pinion shaft is attached and which rotatably supports a pinion gear is integrally joined by welding to a second differential case that does not support the pinion gear. A ring gear is then integrally joined by welding near the portion where the first differential case is joined to the second differential case.
However, when the first differential case and the second differential case are integrally joined together by welding and a ring gear is then integrally joined, by welding to that differential case in this way, two welding operations must be performed, which increases the manufacturing cost. Incidentally, JP-A-2006-509172 does propose to reduce the number of welds by integrally providing the ring gear on the second differential case, but the conditions for the required strength and wear resistance and the like are different for the differential case than they are for the ring gear, so the resulting product may not necessary be entirely satisfactory, e.g., material limitations may, actually lead to higher costs.